1. Field of the Invention
The present invention relates to a zoom finder made of an optical system separate from a photographing lens, and more particularly to a zoom finder in which the magnification is varied while keeping a constant diopter of the finder.
2. Description of Related Art
In a camera having a photographing optical system and a finder optical system separate therefrom, such as a lens shutter type camera or a video camera, if the photographing optical system is comprised of a zoom lens, it is preferable to incorporate a zoom finder in which the magnification of the finder is varied in accordance with the power (magnification) of the photographing optical system.
An inverted Galilean finder is usually used for the zoom finder, as disclosed for example in Japanese Unexamined Patent Publication No. 53-63104. However, it is difficult to observe through an eye piece a virtual image formed by an objective lens in an inverted Galilean finder. Furthermore, there were drawbacks, such as an increase of the diameter of a first lens group (objective lens group) nd a dimmed field frame.
In addition to the foregoing, there is a tendency in a lens shutter type camera or a video camera that a photographing optical system thereof is made of a zoom lens of high power ratio and high efficiency. To respond to such a tendency, there is a long felt need for the development of a zoom lens in which a high variable power ratio can be easily realized and little change of aberration due to the power variation takes place.
However, in a conventional real image type of zoom finder having a high variable power ratio, no aberration can be completely eliminated, and accordingly, a large fluctuation of the aberration by the zooming takes place, thus resulting in a dimmed finder image.
Furthermore, where the photographing optical system is comprised of a zoom lens, the photometering (brightness measuring) range for automatic exposure must be varied in accordance with a change in the photographing range. One solution thereto is to take a light flux of an object to be taken out from an image erecting prism, as disclosed in Japanese Unexamined Patent Publication No. 1-209432.
However, to take the light flux of the object out from the image erecting prism, it is necessary to provide an additional lens or lenses to adjust the photometric sensitivity distribution of measuring, such as a center-weighted measuring or an averaged light measuring.